More details of the code

A error protection code used is known as an RS(59,63) code. This code is based on the use of 6-bit symbols, with 4 parity symbols used to protect 59 data-symbols in a 63-symbol block. The code is able to correct up to two errored symbols in any block.

To add more protection against burst errors, 6-way interleaving was also used to spread the 63 symbols in any block over the 378 symbols of each frame (this is the origin of the 378-word frame length). This is analogous to using 6 error correctors in parallel, each one working on every 6th word of the frame.

At an input bit error rate (BER) of 5x105 (which is 5 random errors in every 100,000 bits), the error-corrector reduces this to an output BER of 1x10 9. The system still gives about an order of magnitude improvement with an input BER of 1x10 3 and only fails to reduce the errors at an input BER of 1x102  - at which point the whole system is close to total failure because of the inability to achieve frame synchronisation in the decoder.

A characteristic of these types of codes is that their performance in respect of burst errors is superior to the worst-case random errors considered above. Error-free transmission is possible even with a burst of 67 bits in every frame a mean error rate of 3x102.